Many image capturing devices (such as cameras and cell phones) employ autofocus systems which can focus on selected points or areas. The autofocus system can be switched between an active autofocus mode and a passive autofocus mode. When the autofocus optical system is under the active autofocus mode, the autofocus system sends some form of energy towards the target object, analyses the reflection to determine a distance between the target object and the active autofocus system, and adjusts the lens according to the distance. When the autofocus optical system is under the passive autofocus mode, the autofocus system analyses the image of the target object without sending any energy towards object.
Thus, the image capturing device can determine the distance between the target object and the autofocus system only when the autofocus optical system is under the active autofocus mode. However, the autofocus system is under the active autofocus mode when the image capturing device is located in an environment with a low illumination. That is, the image capturing device cannot determine the distance between the target object and the autofocus system when the image capturing device is located in an environment with a high illumination.